Chapter 2 Devices 2.2. Device Functions The following flow chart shows the procedure for processing a program in the PLC. Regenerating the input signal 1. Before the program is executed, the state of the Input terminal X Regenerating the input signal external input signal is read into the memory location for the input signal. 2_ Device memory 2. When program is executed, the state in the memory Device memory location for the input signal does not change even if the input signal changes from ON to OFF or from OFF Processing the program to ON. The input signal is not refreshed until the next scan begins.
Device memory Processing the program After the input signal is refreshed, the instructions in the program are executed in order from the start address of the Regenerating the output signal and sending it to the output terminal Regenerating the state of the output program. The results are stored in the device memory. After the instruction END is executed, the state in the device memory is sent to the specified output terminal. 2.2.1 Values and Constants The relation among bits, nibbles, bytes, words, and double words in the binary system is shown in the picture below.
DW Double word W1 W0 Word BY3 BY2 BY1 BY0 Byte NB0 NB1 NB2 NB3 NB4 NB5 NB6 NB7 Nibble Bit 2-5 Name Description Bit A bit is the basic unit in the binary system. Its state is either 1 or 0. Nibble A nibble is composed of four consecutive bits (for example b3–b0). Nibbles can represent 0–9 in the decimal system, or 0–F in the hexadecimal system. Byte A byte is composed of two consecutive nibbles (8 bits, b7–b0). Bytes can represent 00–FF in the hexadecimal system. Word A word is composed of two consecutive bytes (16 bits, b15–b0). Words can represent 0000–FFFF in the hexadecimal system. Double word A double word is composed of two consecutive words (i.e. 32 bits, b31–b0).
Double words represent 00000000–FFFFFFFF in the hexadecimal system. b31 b30 b29 b28 b27 b26 b25 b24 b23 b22 b21 b20 b19 b18 b17 b16 b15 b14 b13 b12 b11 b10 b9 b8 b7 b6 b5 b4 b3 b2 b1 b0